This invention relates to a method for preparing oriented conductive polymer films and, more specifically, to oriented polyaniline.
Because of its excellent chemical stability and the relatively high levels of electrical conductivity of certain forms of the material, much attention has recently been directed to polyaniline. The base form of the polymer in the emeraldine oxidation state (y equals about 0.5) can be represented by the formula ##STR1## which contains equal numbers of alternating reduced, ##STR2## and oxidized, ##STR3## repeat units. The polymer can be protonated (or "doped") by dilute aqueous protonic acids such as HCl to produce the corresponding salt (A=anion): ##STR4## The polymer exhibits conductivities of about 1-5 S/cm when approximately half of its nitrogen atoms are protonated as shown above.
It has been disclosed that free-standing films of emeraldine acetate (conductivity of about 0.5-2 S/cm) can be cast from solutions of emeraldine base in 80% aqueous acetic acid. M. Angelopoulos, A. Ray, A. G. MacDiarmid, and A. J. Epstein, Synth. Met., 21, 21-30 (1987). M. Angelopoulos, G. E. Asturias, S. P. Ermer, A. Ray, E. M. Scherr, A. G. MacDiarmid, Z. Kiss, M. Ahktar, and A. J. Epstein Mol. Cryst. Lig. Cryst., 1988, Vol. 160, pp. 151-163, disclose that the best solvent for producing high quality free-standing films of emeraldine base is 1-methyl-2-pyrrolidinone (NMP), and that the mechanical properties of films cast from NMP are greatly superior to those of films cast from 80% acetic acid as previously described. The films prepared by Angelopoulos et al. can be doped to the metallic conducting regime, 1-5 S/cm.
It has recently been disclosed that polyaniline may be successfully processed into monofilaments and films from solutions in concentrated sulfuric acid. A. Andreatta, Y. Cao, J. C. Chiang, A. J. Heeger and P. Smith, "Electrically Conductive Fibers of Polyaniline Spun from Solutions in Concentrated Sulfuric Acid," Synth Met., 26 p. 383-389 (1988). The polyaniline derived from sulfuric acid is disclosed as displaying sharp X-ray reflections, indicative of a significant degree of crystallinity, and the electrical conductivity of as-spun, washed and dried fibers was in the range from 20-60 S/cm. Andreatta et al. estimate that the molecular weight of the polyaniline used to prepare their fibers is between 12,000 and 40,000.
There are a multitude of important uses for films of conductive materials such as polyaniline. For example, such films can be used as electrodes in light-weight rechargeable batteries, as sensors to detect HCl, NH.sub.3, NO.sub.2, H.sub.2 S and other toxic gases, as dosimeters (detectors) for nuclear radiation, as electromagnetic shields and as microwave absorbing materials. The films would find even greater utility if there were a means for increasing their level of conductivity.